I am working on prediction of flow around a simple car body. I have created a box and placed the car body within this box and applied a sizing function between the car faces and the box faces. As I see it there are two options for the outer boundary conditions (i.e the side walls of the box)...pressure outlet or symmetry. When I use the symmetry boundary condition the computation quickly converges, which makes sense. When I use pressure, the residuals do not steadily decrease and convergence is not as good. I also receive a notification that the turbulent viscosity ratio is limited to a value when I run with the pressure boundary condition. I feel as though the pressure B.C. is the most appropriate in this case... is this correct? What can I do to improve convergence when using the pressure B.C.?

Personally I have never seen external aero run with pressure BC round the far field, I always use symmetry and have always seen other people use symmetry, if the boundaries are far enough away from the object of interest there should be no problem at all. Why do you feel pressures are correct?

Thanks for the reply. I am with you in that I have always seen symmetry used for external flow. I also understand that as the boundaries (symmetry) are moved away from the object the symmetry condition has less of an effect on the solution. I have run some simple flows i.e. flow over a flat plate, jet, wake, etc. in which I have compared the CFD results to analytical results. What I have found is that when symmetry is used the results are close but don't exactly match the analytical solution. However when I use pressure-outlet the results do closely agree with the analytical solution. I know that an exact match doesn't exist but from what I have seen soltions can be very close to analytical or experiment. It seems that the real issue (when using the pressure boundary condition) is the proper meshing of the geometry. I am using this reasoning to conclude that the pressure B.C. is the correct choice. I have also heard people talk of starting out with symmetry and then switch to pressure. I have never been able to get this method to work...perhaps it is just talk.

When the side wall is pretty far from the body,the effect of the far field condition on the can be negligible.The effect of the sidewall scales ~ 1/Re^0.5 and hence the effect of the sidewall will not be felt.

You can look the definition of the symmetry BC in any CFD book (e.g peric ) and from your computational domain description, I guess you are doing the calculations for at least 5 cars of the same size moving at the same speed, is that your intention?

As a starter, read about the exact mathematical solution of Rayleigh first problem, then try some experimental work, very easy in your case, stand on the side walk of any street, use ear plugs and a pair of sun glasses so you can not see or hear the passing cars, then try to feel the passing cars by noticing the effect of air moving around you as a result of these passing cars, that will give you a good idea of how far your computational domain would extend in the normal direction, As you say, the convergence when you specify the sides as symmetry BC compared to the pressure BC is much faster, do you know why? Could you tell us the BC you specified on the top of your computational domain and Why? Good Luck